Systems and methods for reading data

ABSTRACT

A system for reading data is described. The system includes a detector that includes a first semiconductor device and a second semiconductor device. The second semiconductor device is configured to remove a need to scrub the detector.

BACKGROUND OF THE INVENTION

This invention relates generally to imaging systems and more particularly to systems and methods for reading data from a detector.

A solid state x-ray detector includes of an array of pixels having a plurality of switches and photodiodes over which Cesium Iodide (CsI) is deposited. The CsI absorbs x-rays and converts the x-rays to light, which is detected by the photodiodes. Each photodiode, due to its construction, acts as a capacitor and stores charge. Initialization of the detector takes place prior to an x-ray exposure, when each photodiode is charged to an initial voltage. The detector is then exposed to the x-rays, which are absorbed by the CsI. The light that is emitted in proportion to a portion of the x-rays partially discharges each photodiode. After the exposure, a voltage on each photodiode is restored to the initial voltage. An amount of charge used to restore the initial voltage on the photodiode is measured, which becomes a measure of an x-ray dose integrated by a pixel during a length of the exposure.

The detector is read or alternatively scrubbed on a row-by-row basis, as controlled by the switches associated with each photodiode. Reading is performed whenever an image acquired by the detector includes exposure data or alternatively offset data. Scrubbing is similar to reading except that data acquired from scrubbing is not interesting, and is therefore discarded. Scrubbing is performed to maintain proper bias on the photodiodes during idle periods, perhaps to reduce a plurality of effects of lag, which is incomplete charge restoration of the photodiodes, or alternatively to maintain a plurality of thresholds of the switches. The thresholds may shift if the switches are kept in an “off” state for long periods, among other reasons. Scrubbing restores charge on each photodiode and the charge need not be measured. If the charge is measured, data acquired from scrubbing can be discarded.

Scrubbing is performed to keep the detector ready for use largely due to the less than ideal characteristics of amorphous silicon used to fabricate the detector.

However, scrubbing is undesirable for several reasons. Scrubbing represents non-productive overhead, uses power to perform, the dissipation of which is undesirable especially in low power applications. Scrubbing may create an access time latency before the commencement of the exposure to allow completion of the scrub prior to the start of the exposure. The time used to scrub takes away from the detector's availability.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a system for reading data is described. The system includes a detector that includes a first semiconductor device and a second semiconductor device. The second semiconductor device is configured to remove a need to scrub the detector.

In another aspect, an imaging system for reading data is described. The imaging system includes an energy source configured to generate energy that passes through a subject, and a detector configured to receive a portion of the energy. The detector includes a first semiconductor device and a second semiconductor device and the second semiconductor device is configured to remove a need to scrub the detector.

In yet another aspect, a method for reading data is described. The method includes removing a need to scrub a first detector by coupling a first semiconductor device to a second semiconductor device.

In still another aspect, a method for removing image artifacts is described. The method includes coupling a first semiconductor device to a second semiconductor device and a photodiode, and during an activation of the second semiconductor device, applying a potential to the second semiconductor device that is more negative than a potential applied to an anode of the photodiode. The method further includes during the activation of the second semiconductor device, applying a potential to the second semiconductor device similar to a potential of a data line attached to the first semiconductor device subsequently after applying the potential to the second semiconductor device that is more negative than the potential applied to the anode of the photodiode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of an imaging system.

FIG. 2 is a block diagram of another embodiment of an imaging system.

FIG. 3 is a block diagram of an embodiment of a detector of the imaging system of FIG. 2.

FIG. 4 is a circuit diagram of an embodiment of a photo detector array of the detector.

FIG. 5 is a timing diagram illustrating an embodiment of a method of reading data from the photo detector array of FIG. 4.

FIG. 6 is a circuit diagram of another embodiment of a photo detector array of the detector.

FIG. 7 is a timing diagram illustrating an embodiment of a method of reading data from the photo detector array of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of an embodiment of an imaging system 14. Imaging system 14 includes source 15, such as an x-ray tube or a gamma ray source, which, when excited by a power supply 16, emits a beam 17, such as an x-ray beam or a gamma ray beam. The beam 17 is directed toward a subject 18, such as a patient or a phantom, lying on a transmissive table 20. A portion of the beam 17 which is transmitted through the transmissive table 20 and the subject 18 impinges upon a detector 22, such as an x-ray or a gamma ray detector. Detector 22 includes a scintillator layer 24 that converts a plurality of higher energy photons of the portion of beam 17 to lower energy photons. The lower energy photons have energies lower than the higher energy photons. Contiguous with the scintillator layer 24 is a photo detector array 26, which converts the lower energy photons into a plurality of electrical signals. A detector controller 27 includes electronics that operates photo detector array 26 to acquire an image, such as an x-ray image or a gamma ray image, and to read an electrical signal from each photo detector element of photo detector array 26. Examples of the x-ray image include a radiographic image, an image of a chest of subject 18, an angiographic image, a cardio graphic image, and a mammographic image.

The electrical signals from the photo detector array 26 are coupled to an image processor 28 that includes circuitry that processes and enhances, such as amplifies or filters, the electrical signals to generate the image. As an example, image processor 28 might apply image reconstruction, such as filtered backprojection (FBP) or maximum intensity projection (MIP) to generate a computed tomography (CT) image or other algorithms to generate diagnostic images from the electrical signals. As used herein, the term controller is not limited to just those integrated circuits referred to in the art as a controller, but broadly refers to a processor, a computer, a microcontroller, a microcomputer, a programmable logic controller, an application specific integrated circuit, a Field Programmable Gate Array, and any other programmable circuit. Moreover, as used herein, the term processor is not limited to just those integrated circuits referred to in the art as a processor, but broadly refers to a controller, a computer, a microcontroller, a microcomputer, a programmable logic controller, an application specific integrated circuit, a Field Programmable Gate Array, and any other programmable circuit.

The image is displayed on a monitor 32 and may be archived in an image storage device 30. Examples of image storage device 30 include a computer-readable medium, such as a hard drive, a volatile memory, a non-volatile memory, a floppy disk, a compact disc—read only memory (CD-ROM), a magneto-optical disk (MOD), and a digital versatile disc (DVD). Examples of monitor 32 include a cathode ray tube (CRT) and a liquid crystal display (LCD). The image processor 28 additionally produces a brightness control signal which is applied to an exposure control circuit 34 to regulate the power supply 16 and to reduce or alternatively increase an exposure of the portion of beam 17 on detector 22. The overall operation of imaging system 14 is governed by a system controller 36 that receives commands from a technician or a person via an operator interface panel 38, such as a mouse or a keyboard.

FIG. 2 is a block diagram of another embodiment of an imaging system 50. Imaging system 50 includes imaging system 14. Moreover, imaging system 50 includes a source 52 and a detector 54. Source 52 can be an x-ray source or a gamma ray source. Detector 54 includes a scintillator layer 56 and a photo detector array 58. Source 52 is located at an angle, such as ranging from and including 1 degrees to 179 degrees, with respect to source 15. Moreover, detector 54 forms an angle, such as ranging from and including 1 degrees to 179 degrees, with respect to detector 22. Source 52 and detector 54 are used to acquire to a view of subject 18 different than a view acquired by source 15 and detector 22.

Source 52 generates a beam 60, such as an x-ray beam or a gamma ray beam, directed toward subject 18 when source 52 is excited by power supply 16. Upon passing through subject 18, a portion of beam 60 is transmitted towards detector 54. Scintillator layer 56 converts a plurality of higher energy photons of the portion of beam 60 to a plurality of lower energy photons having energies lower than the higher energy photons. The lower energy photons of the portion of beam 60 lie within a spectrum that can be detected by the photo detector array 58. Photo detector array 58 converts the lower energy photons of the portion of beam 60 into a plurality of electrical signals. Image processor 28 reads the electrical signals from detector 54, and processes and enhances the electrical signals to generate an image, such as an x-ray image or a gamma ray image. For example, image processor 28 applies image reconstruction, such as FBP or MIP or other image processing algorithms, to the electrical signals received from photo detector array to generate the image. The image generated from the electrical signals output by detector 54 is stored in image storage device 30. Image processor 28 produces a brightness control signal that is applied to exposure control circuit 34 to regulate power supply 16 and an exposure of the portion of beam 60 incident on detector 54.

FIG. 3 is a block diagram of an embodiment of a detector 70, which is an example of any of detectors 22 and 54. Detector 70 includes a scintillator layer 72, fabricated from a scintillator, such as cesium iodide (CsI). Detector 70 includes a photo detector array 74 and a substrate 76. Photo detector array 74 is an example of one of photo detector array 26 and photo detector array 58. Scintillator layer 72 is an example of one of scintillator layer 24 and scintillator layer 56.

Scintillator layer 72 absorbs the portion of one of the beams 17 and 60 to convert the higher energy photons of the portion of the one of beams 17 and 60 into the lower energy photons of the portion of the one of beams 17 and 60. Photo detector array 74 receives the lower energy photons of the portion of the one of beams 17 and 60 to generate a plurality of electrical signals. Substrate 76 supports photo detector array 74 and scintillator layer 72.

FIG. 4 is a circuit diagram of an embodiment of a photo detector array 80, which is an example of photo detector array 74. Photo detector array 80 includes a plurality of scan lines 82 and 84, and a plurality of data lines 86 and 88. Photo detector array 80 is formed by a matrix of pixels or detector elements 90. Detector elements 90 are arranged on substrate 76. Each detector element 90 includes a photodiode 92 made of a material, such as silicon. Examples of silicon include amorphous silicon and crystalline silicon. Moreover, each detector element 90 includes a thin film field effect transistor (FET) 94. The photodiode 92 is fabricated over a large portion of detector element 90 in order that the photodiode 92 will intercept a sizeable portion of the light produced by scintillator layer 72. Each photodiode 92 has a capacitance that allows the photodiode 92 to store an electrical charge, which is then partially or alternatively wholly discharged due to an excitation by the lower energy photons of the portion of one of beams 17 and 60.

The cathode of each photodiode 92 in each detector element 90 of each column of the photo detector array 80 is connected via a source-drain conduction path of the FET 94 to one of data lines 86 and 88. Data lines 86 and 88 are connected to a plurality of sensing circuits 96 and sensing circuits 96 maintain data lines 86 and 88 at a constant potential at all times. The sensing circuits 96 are included in the image processor 28. The anode of each photodiode 92 is connected to a common electrode 98. A gate electrode of FET 94 in each row is connected to one of scan lines 82 and 84. Each scan line 82 and 84 runs the full dimension of detector 70. Scan lines 82 and 84 are coupled to the detector controller 27. In another embodiment, photo detector array 80 is formed of any integer, m, of scan lines and any integer, n, of data lines.

FIG. 5 is a timing diagram 110 illustrating an embodiment of a method of reading data from a detector. To acquire an image, such as an x-ray image or a gamma ray image, by using detector 70, initially, detector 70 is scrubbed 112. Scrubbing 112 may be performed to maintain a known potential or a known voltage on the photodiodes 92 during idle periods, to reduce a plurality of effects of image retention or lag, and/or to protect a plurality of operating characteristics of the FETs 94. Sensing circuits 96 restore charge of the photodiodes 92 of detector elements 90 of detector 70 during scrubbing 112. One of scan lines 82 and 84 being scrubbed 112 operates detector elements 90 connected to a corresponding one of data lines 86 and 88. For example, detector element 90 connected to data line 86 is operated to scrub 112 scan line 82. During scrub 112, a high negative voltage, such as −a volts, is applied to the common electrode 98 by a power source (not shown), where a is a positive real number other than zero. The sensing circuits 96 apply a low negative voltage, such as −b volts, to data lines 86 and 88 that is lower than the high negative voltage, where b is a positive real number. The number b is lower than the number a.

Moreover, during scrub 112, detector controller 27 switches scan lines 82 and 84 from a voltage, such as −c volts, more negative than the high negative voltage of common electrode 98 to a positive voltage, causing the FETs 94 attached to scan lines 82 and 84 to begin to conduct, where c is a positive real number. The number c is higher than the number a. Detector controller 27 includes a plurality of drive circuits to drive or provide power to scan lines 82 and 84. During scrub 112, the photodiode 92 continues to store charge until a voltage across the photodiode 92 is equal to a voltage difference between a corresponding one of data lines 86 and 88 and common electrode 98 and until the photodiode 92 is charged to the known voltage, after which the FETs 94 are switched off. For example, the photodiode 92 continues to store charge until a voltage across the photodiode 92 is equal to a voltage difference between data line 88 and common electrode 98. To end scrub 112, the FETs 94 are switched off by detector controller 27, which reapplies, to scan lines 82 and 84, a potential, such as −c or −d volts, that is more negative than the high negative voltage of common electrode 98, where d is a positive real number. The number d is higher than the number a. Image data used to produce an image, such as an x-ray or a gamma ray image, is not acquired during scrub 112.

Before system controller 28 controls power supply 16 to activate one of sources 15 and 52, system controller 28 makes an exposure request 114. Upon receiving exposure request 114, detector controller 27 determines whether scrub 112 has ended. Upon determining that scrub 112 has not ended, detector controller 27 does not grant 116 exposure request 114 received from system controller 28. On the other hand, upon determining that scrub 112 has ended, detector controller 27 grants 116 exposure request 114 received from system controller 28. A scrub latency 118, which is a time difference between a start of exposure request 114 and a start of grant 116 of exposure request 114 is developed. Detector 70 is exposed 120 to the portion of one of beams 17 and 60, which is controlled by an amount of power supplied to a corresponding one of sources 15 and 52. Detector 70 is exposed 120 during grant 116 of exposure request 114.

When the lower energy photons of the portion of one of beams 17 and 60 strike photodiode 92, the photodiode 92 conducts and a capacitance of photodiode 92 is partially discharged. An amount of charge removed from the capacitance of photodiode 92 depends upon an amount of the lower energy photons of the portion of one of beams 17 and 60, and the amount depends upon an intensity and duration of the portion of one of beams 17 and 60 that strikes scintillator layer 72 during exposure 120.

Upon termination of the exposure 120 of detector 70 to the portion of one of beams 17 and 60, a charge in each photodiode 92 is restored to the known voltage before the exposure. The exposure 120 of detector 70 to the portion of one of beams 17 and 60 terminates when system controller 28 controls power supply 16 to discontinue supplying power to one of sources 15 and 52.

Upon termination of the exposure 120 of detector 70 to the portion of one of beams 17 and 60, detector controller 27 reads 122 detector 70 by sequentially applying a positive voltage to scan lines 82 and 84. The image data is acquired by sensing circuits 96 during read 122. When one of scan lines 82 and 84 is positively biased, the FETs 94 connected to the one of scan lines 82 and 84 are turned on to couple the corresponding photodiodes 92 in the selected row to a corresponding one of data lines 86 and 88. For example, when scan line 82 is positively biased, the FET 94 connected to the scan line 82 is turned on to couple the photodiode 92 to data line 86. An amount of charge used to restore the voltage difference between the one of data lines 86 and 88 and common electrode 98 to the known voltage is measured by the sensing circuits 96.

Detector 70 is scrubbed 112 after detector 70 is read 122. Sensing circuits 96 restore charge, if necessary, to photodiode 92 during scrub 112 in order to restore the potential across the photodiode 92. If sensing circuits 96 measure the charge used to restore the voltage across photodiode 92 during scrub 112, the measurement is discarded.

FIG. 6 is a circuit diagram of an embodiment of a photo detector array 130, which is another example of photo detector array 74. Photo detector array 130 includes scan lines 82 and 84, data lines 86 and 88, FETs 94, and photodiodes 92. Moreover, photo detector array 130 includes a plurality of FETs 132, such as an N-type metal-oxide semiconductor FET. A source electrode of FET 94 is coupled to a source electrode of FET 132. Moreover, a drain electrode of FET 132 is coupled to a maintenance potential electrode 134 and a gate electrode of FET 132 is coupled to a maintenance control electrode 136. Moreover, the source electrode of FET 132 is coupled to the cathode of photodiode 92.

Photodiodes 92, FET 94, and FET 132 form a pixel 138. Maintenance control electrode 136 is electrically connected to a power supply 140, such as a voltage source. Moreover, maintenance potential electrode 134 is electrically coupled to a power supply 142, such as a voltage source.

It is noted that in another embodiment, another type of device, such as a bipolar junction transistor (BJT), can be used, with a selection of supply voltages 140 and 142, instead of FET 132. For example, a base of the BJT is coupled to maintenance control electrode 136, an emitter of the BJT is coupled to maintenance potential electrode 134, and a collector of the BJT is coupled to the cathode of photodiode 92 and the source electrode of FET 94. Examples of the BJT include an NPN BJT and a PNP BJT. Moreover, other types of devices, such as a P-type MOSFET, a Junction FETs (JFETs), metal-semiconductor FET (MESFETs), or a diode, can be used instead of FET 132. In another embodiment, photo detector array 130 includes any number of pixels 138.

FIG. 7 is a timing diagram 150 illustrating an embodiment of a method of reading data from photo detector array 130. Power supply 140 applies a voltage 152 to maintenance control electrode 136 to maintain a bias across photodiode 92. A magnitude of voltage 152 applied to FETs 132 to activate or turn on FETs 132 is more positive, such as e volts, than the low negative voltage, such as −b volts, applied by the sensing circuits 96 to data lines 86 and 88, and the more positive voltage 152 is applied to maintain bias across photodiode 92, where e is a positive real number. The number e is higher than 0, which is greater than −b. The higher the voltage e applied to maintenance control electrode 136 of FETs 132, the “harder” FETs 132 will turn “on”, i.e. with lower impedance. A voltage applied to maintenance potential electrode 134 from power supply 142 is similar to a voltage applied to at least one of data lines 86 and 88 to maintain bias across photodiode 92. For example, the voltage applied to maintenance potential electrode 134 ranges from and including a voltage applied to data line 86 to a voltage applied to common electrode 98. As another example, when a voltage applied to data line 86 is −f volts and a voltage applied to common electrode 98 is −g volts, the voltage applied to maintenance potential electrode 134 ranges from and including −f volts to −g volts, where f and g are real and positive numbers and f is less than g. As another example, the voltage applied to maintenance potential electrode 134 ranges from and including a voltage applied to data line 88 to a voltage applied to common electrode 98. As yet another example, when a voltage applied to data line 88 is −f volts and a voltage applied to common electrode 98 is −g volts, the voltage applied to maintenance potential electrode 134 ranges from and including −f volts to −g volts. Photodiode 92 charges when the bias is maintained across photodiode 92.

Moreover, the bias across photodiode 92 is maintained at a time at which sensing circuits 96 and detector controller 27 are deactivated, turned off, or not powered by a power supply (not shown) within detector 70. Additionally, the bias across photodiode 92 is maintained at a time during which detector 70 is at least one of not exposed 120, not read 122, and not integrating offset data. Sensing circuits 96 are controlled by system controller 36 to prevent sensing circuits 96 from reading 122 from detector 70 until a certain time. The prevention until the certain time integrates offset data.

When exposure request 114 is received from system controller 28, detector controller 27 is not scrubbing 112 and does not need to wait to finish scrubbing 112 before granting 116 exposure request 114. When exposure request 114 is received from system controller 28, power supply 140 reduces voltage supplied to gate electrodes of FETs 132 to −c or alternatively −d volts to deactivate or turn off FETs 132. By turning off FETs 132, the bias is discontinued to be maintained across photodiode 92 and photodiode 92 discontinues to be charged by maintenance potential electrode 134. When the bias is discontinued to be maintained across photodiode 92, detector controller 27 grants 116 exposure request 114 received from system controller 28. At an end of exposure request 114, detector 70 is read 122 by sensing circuits 96. Detector 70 is read 122 by sensing circuits 96 and scrub 112 is not performed by using FETs 132. A non-scrub latency 154, which is a time difference between a start of exposure request 114 and a start of grant 116 of grant 116 of exposure request 114 is less than scrub latency 118.

In one embodiment, the voltage applied to maintenance potential electrode 134 is equal to a value at one of data lines 86 and 88. In another embodiment, when the bias is maintained across photodiode 92 of detector 54, detector 22 is exposed to the portion of beam 17. After exposure of detector 22 to the portion of beam 17, the bias is discontinued to be maintained across photodiode 92 of detector 54. Moreover, when the bias is discontinued to be maintained across photodiode 92 of detector 54, the detector 54 is exposed to the portion of beam 60. After exposure of detector 54 to the portion of beam 60, detectors 22 and 54 are read 122 by sensing circuits 96. It is noted that exposing detector 22 during maintenance of the bias across photodiode 92 of detector 54 reduces, such as eliminates, the effect of scatter, generated by the portion of beam 17, on detector 54.

It is further noted that offset data is read by sensing circuits 96 from detector 70 in a similar, such as the same, manner in which the image data is read from detector 70 during read 122 except that there is no exposure 120 made. Image Processor 28 subtracts the offset data from the image data to account for any non-zero signal contribution outside of exposure signal, such as diode leakage and any differences between a potential of maintenance potential electrode 134 and a potential of any one of data lines 86 and 88.Moreover, during activation or turning on of FET 132, artifacts are removed by applying to maintenance potential electrode 134, the voltage that is more negative, such as −h volts, than a voltage, such as −i volts, applied to common electrode 98, where h and i are positive real numbers. The number h is higher or greater than the number i. Additionally, during the activation of FET 132, artifacts are removed by applying to FET 132 a voltage that is similar to a potential of data line 86 attached to transistor FET 94 subsequently after applying to maintenance potential electrode 134, the voltage that is more negative, such as −h volts, than a voltage, such as −i volts, applied to common electrode.

Technical effects of the herein described systems and methods for reading data include reducing a time and power utilized to scrub 112 detector 70. Moreover, removing a need to scrub 112 detector 70 also results in non-scrub latency 154 being less than the scrub latency 118. Power is limited when detector 70 is portable and is powered by a battery. The power utilized by sensing circuits 96 to scrub 112 detector 70 is saved when detector 70 is not scrubbed.

While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims. 

1. A method for reading data using an imaging system, comprising a detector that comprises a first semiconductor device, a second semiconductor device, and a photodiode, the first semiconductor device and the second semiconductor device coupled to the photodiode, said method comprising: applying a first voltage to at least one data line coupled to the first semiconductor device using sensing circuits coupled to the at least one data line, the sensing circuits configured to maintain the at least one data line at a substantially constant potential; applying a second voltage to a maintenance control electrode coupled to the second semiconductor device to maintain a bias across the photodiode, the first semiconductor device coupled to the photodiode; applying a third voltage to a maintenance potential electrode coupled to the second semiconductor device to maintain the bias across the photodiode; deactivating the second semiconductor device after receiving an exposure request, wherein deactivating the second semiconductor terminates the application of the second voltage and the third voltage; exposing the detector to radiation; reading image data from the detector after the exposure has terminated, the image data read via at least one scan line coupled to the first semiconductor device; and reapplying the second voltage and the third voltage.
 2. A method for reading data in accordance with claim 1, wherein applying a second voltage and a third voltage to maintain the bias across the photodiode further comprises maintaining the bias when the detector is at least one of not being read, not exposed to x-rays, and not allowed to integrate an offset signal.
 3. A method for reading data in accordance with claim 1, wherein applying a second voltage to the maintenance control electrode further comprises applying a potential to a gate electrode of the second semiconductor device; and applying a third voltage to the maintenance potential electrode further comprises applying a potential to a drain electrode of the second semiconductor device.
 4. A method for reading data in accordance with claim 1, wherein deactivating the second semiconductor device after receiving an exposure request further comprises: receiving the exposure request; granting the exposure request after a non-scrub latency; and reducing the second voltage to deactivate the second semiconductor device.
 5. A method for reading data in accordance with claim 1, wherein applying a third voltage to a maintenance potential electrode further comprises applying a third voltage that is substantially equal to the first voltage.
 6. A method for reading data in accordance with claim 1 further comprising: reading offset data from the detector using the sensing circuits; and subtracting the offset data from the image data to account for contributions from signals other than signal produced by the exposure of the detector to the radiation.
 7. A method for removing image artifacts using an imaging system, the imaging system comprising a first detector including a first photodiode that is configured to be exposed to a first radiation beam, a second detector including a second photodiode configured to be exposed to a second radiation beam, wherein the first photodiode is coupled to a first semiconductor device and a second semiconductor device and the second photodiode is coupled to a third semiconductor device and a fourth semiconductor device, said method comprising: applying a first voltage to at least one data line coupled to the first semiconductor device and to at least one data line coupled to the third semiconductor device using sensing circuits, the sensing circuits configured to maintain the data lines at a substantially potential; applying a second voltage to maintenance control electrodes coupled to the second semiconductor device and the fourth semiconductor device to maintain a bias across the first photodiode and a bias across the second photodiode; applying a third voltage to maintenance potential electrodes coupled to the second semiconductor device and the fourth semiconductor device to maintain the bias across the first photodiode and the bias across the second photodiode; maintaining the bias across the first photodiode using the second semiconductor device while deactivating the third semiconductor device upon receiving an exposure request; exposing the second photodiode to the second radiation beam while the bias across the first photodiode is maintained; deactivating the second semiconductor device coupled the first photodiode after exposing the second photodiode to the second radiation beam; exposing the first photodiode to the first radiation beam; reading data from at least one scan line coupled to the first photodiode and from at least one scan line coupled to the second photodiode; and reapplying the second voltage and the third voltage to the second semiconductor device and the fourth semiconductor device. 